With an increase in the density of electronic components mounted on printed wiring boards in recent years, the method of mounting IC packages on the wiring board is changing over from the lead insertion method wherein a plurality of leads are inserted into holes in the wiring board predominantly to the surface mounting method wherein the outer ends of leads are soldered directly to pads (conductor pattern) formed on the surface of the wiring board.
Widely known as electronic components to be mounted on the surface of wiring boards, i.e., as surface mount components, are IC packages, such as SOPs (Small Outline Packages) or QFPs (Quad Flat Packages), which comprise an IC chip enclosed or sealed with a molded resin in the form of a package and a plurality of leads formed from a lead frame.
With IC packages, the lead pitch is becoming ever smaller as the number of pins on IC cips increases. QFPs with a lead pitch of 0.5 mm are made available recently, and further development efforts are devoted to realize lead pitches of not greater than 0.3 mm.
Under such a situation, attention has been directed to TCPs (Tape Carrier Packages) which are produced by the TAB (Tape Automated Bonding) technique.
FIGS. 63 and 64 show a TCP which comprises an IC chip 1 connected to a plurality of leads 30 formed on a film carrier tape 400, with the connections enclosed with a resin 2. Each lead has an inner lead portion 31 which is joined to a metal projection, i.e., bump 11, formed on the rear surface of the IC chip 1 in advance.
With the TCP, the plurality of leads 30 are formed by etching a copper foil affixed to the carrier tape 400, so that the lead pitch can be made smaller than is the case with QFPs and the like.
When to be mounted on the surface of a wiring board 7, the TCP is placed on the board 7 with outer lead portions 32 in register with a solder layer 73 precoating the wiring board 7, and the outer lead portions 32 are soldered to the board 7 by depressing and heating the lead portions 32 with thermocompression bonding heads 81 and thereby melting the solder layer 73.
The outer lead portion 32 is depressed by the bonding head 81 because with the TCP, the lead 30 is made of copper foil, therefore does not itself has strength like the lead of the conventional QFP, is supported only at one end, i.e., at its base end, will be positioned as deflected or raised locally or entirely and is likely to be soldered improperly.
Thus, surface mounting of the TCP essentially requires the thermocompression bonding step for the outer lead portions of the leads, consequently necessitating additional equipment including the bonding heads and maintenance of the equipment, whereas it is impossible to mount the TCP on a common wiring board along with various other surface mount components simultaneously by a collective reflow process, hence the problem that the collective reflow equipment is not usable which is predominantly employed in the prior art.
On the other hand, the so-called COG (Chip-On-Glass) technique is known for mounting the IC chip directly on the surface of the wiring board by means of bumps formed on the body of the IC chip in advance. Electronic circuits of higher density can be designed by this technique.
Nevertheless, the CPG technique also requires additional equipment including a clean room for surface mounting and maintenance thereof. Further because the IC chip body has no leads, it is impossible to subject the chip body to various tests, with the result that the packages obtained will include rejects. The joint between the IC chip and the wiring board is positioned on the underside of the chip, so that it is impossible to visually check the joint meticulously with an up-to-date camera to ensure the desired quality. This gives rise to a need to follow a cumbersome procedure for repairing or changing components. The technique is not amenable to the batch reflow process.